1. Field of the Invention
The present invention relates to an image forming apparatus having the function of forming an image on a recording material like a sheet, such as a copying machine, a printer, or a facsimile apparatus.
2. Description of the Related Art
Conventionally, an image forming apparatus of this type is equipped, for example, with a cassette provided in the apparatus main body and serving as a sheet loading means, and a manual feed tray provided, for example, on a main-body side surface and serving as a means for loading special paper sheets like cardboard sheets; sheets are conveyed to a registration roller pair by a sheet feeding means provided in each of the cassette and the manual feed tray, and are conveyed from the registration roller pair to an image forming portion with a controlled timing to thereby effect image formation.
The image forming portion, the conveying portion extending from the cassette to the registration roller pair, and the registration roller pair are driven by a single driving motor, and ON/OFF driving control is performed as needed by using an electromagnetic clutch or the like.
The sheet material fed from the cassette is conveyed to the registration roller pair by a conveying roller and its forward end abuts the registration roller pair to temporarily stop there. Then, the electromagnetic clutch for ON/OFF-controlling the driving of the registration roller pair in conformity with image formation timing is turned ON to drive the registration roller pair, and the sheet material is conveyed toward the image forming portion.
In the image forming portion, the sheet material is sequentially conveyed through the transfer portions of a plurality of photosensitive drums arranged in parallel, whereby developed toners on the photosensitive drums are sequentially transferred to the sheet material, thereby forming a color image on the sheet material. After the completion of the transfer, the transferred image is fixed to the sheet material by a fixing device arranged on the downstream side, whereby a sheet material with an image recorded thereon is obtained.
In the case of a special kind of sheet, such as a cardboard sheet or OHP sheet, the requisite quantity of heat for the fixing is relatively large. In view of this, the conveying speed of the fixing device is lowered to increase the requisite time for the sheet material to pass, whereby the quantity of heat imparted to the sheet is increased, thereby performing fixing in a stable manner.
However, the related art described above involves the following problems.
When an apparatus as described above is reduced in size, the distance between the fixing device and the transfer portion, which constitutes the image forming portion, is reduced, so that it is necessary to make the sheet material conveying speed in the fixing portion substantially equal to the sheet material conveying speed in the transfer portion. Further, it is also necessary to set the conveying speed in the transfer portion to be substantially equal to that in the registration roller pair.
Thus, it is necessary to perform conveyance at a substantially fixed conveying speed in the section from the sheet feeder to the registration roller pair and further, until the sheet has passed through the fixing device.
As a result, the conveying speed in the fixing portion becomes rather low, and more time is required for the output of a first sheet, resulting in a rather poor productivity.
As compared with monochrome printing, only using a toner of one color, this tendency is more conspicuous in the case of full color printing, in which images are recorded by using toners of four colors, since the amount of toner placed on the sheet is then so much the larger.
Further, since the registration roller pair and the image forming portion are driven by the same motor, it can happen that the shock when the sheet bumps into the registration roller pair or when it leaves the registration roller pair is transmitted to the image forming portion being driven, resulting in disturbance in the image.
The present invention has been made with a view toward solving the above problems in the related art. It is accordingly an object of the present invention to provide an image forming apparatus which helps to realize a reduction in apparatus size and an improvement in productivity and makes it possible to obtain a satisfactory image.
In a first aspect of the present invention, there is provided an image forming apparatus comprising: an image forming means including an image bearing member for bearing a toner image, and a sheet material supporting member for supporting a sheet material to which the toner image is transferred, and adapted to hold and convey the sheet material between the image bearing member and the sheet material supporting member to thereby transfer the toner image on the image bearing member to the sheet material; a sheet material conveying means for conveying the sheet material toward the image forming means; a registration means for correcting skew of the sheet material conveyed by the sheet material conveying means and conveying the sheet material to the image forming means with a predetermined timing; an image formation driving source for driving the image forming means; a conveyance system driving source for driving the sheet material conveying means and the registration means; and a control means which makes the speed at which the sheet material is conveyed by the sheet material conveying means variable and which controls the conveyance system driving source and the image formation driving source such that the speed at which the sheet material is conveyed by the registration means is substantially the same as the speed at which the sheet material is conveyed by the image forming means.
In accordance with the present invention, it is possible to independently set the speed at which conveyance is effected by the image forming means and the speed at which conveyance to the registration means is effected, so that it is possible to convey the sheet material at a speed which does not depend on the speed at which image formation is effected, and the interval of the sheet materials to be conveyed is reduced, whereby it is possible to achieve an improvement in terms of productivity. That is, it is possible to provide a high-quality image forming apparatus which is of a simple construction and reduced in size and which helps to realize an improvement in productivity.
Further, the impact generated when the sheet material enters the registration means and when it leaves the registration means is not transmitted to the image formation driving source, so that no disturbance is generated in the image, making it possible to obtain a satisfactory image.
In a second aspect of the invention, it is desirable that the control means control the conveyance system driving source such that the sheet material is conveyed to the image forming means by the registration means at a speed different from the speed at which the sheet material is conveyed to the registration means by the sheet material conveying means.
In a third aspect of the invention, it is desirable that the control means control the conveyance system driving source and the image formation driving source such that the sheet material is conveyed to the registration means by the sheet material conveying means at a speed higher than the speed at which the sheet material is conveyed by the image forming means.
In a fourth aspect of the invention, it is desirable that the control means control the conveyance system driving source such that after the forward end of the sheet material conveyed to the registration means abuts the registration means, the speed at which the sheet material is conveyed by the sheet material conveying means is changed from a first conveying speed to a second conveying speed which is lower than the first conveying speed, and that after the rear end of the sheet material has left the registration means, the speed at which the sheet material is conveyed by the sheet material conveying means is changed from the second conveying speed to the first conveying speed.
In a fifth aspect of the invention, the image forming apparatus further comprises a first drive transmitting means and a second drive transmitting means for transmitting a drive from the conveyance system driving source to the sheet material conveying means at different transmission ratios, and a transmission ratio switching means for switching drive transmission between the first drive transmitting means and the second drive transmitting means, and it is desirable that the control means control the transmission ratio switching means such that when the sheet material is conveyed to the registration means, the drive of the conveyance system driving source is transmitted to the sheet material conveying means through the first drive transmitting means, and that when the sheet material is conveyed to the image forming means by the registration means, the drive of the conveyance system driving source is transmitted to the sheet material conveying means through the second drive transmitting means.
In a sixth aspect of the invention, it is desirable that the transmission ratio switching means be an electromagnetic connecting means for electromagnetically connecting and cutting off the transmission of a rotation drive force, wherein the first drive transmitting means is a rotation drive transmitting member the transmission of which is connected by the electromagnetic connecting means, and wherein the second drive transmitting means is a rotation drive transmitting member which includes a one-way connecting means for exclusively transmitting rotation in one direction and whose reduction ratio is larger than that of the first drive transmitting means.
In a seventh aspect of the invention, it is desirable that the control means control the image formation driving source such that the sheet material is conveyed by the image forming means at two or more conveying speeds.
In an eighth aspect of the invention, the control means preferably has the function of varying the speed at which the sheet material is conveyed by the image forming means according to the kind of sheet material.
In a ninth aspect of the invention, the image formation driving source preferably includes different image formation driving sources respectively driving the image bearing member and the sheet material supporting member.
In a tenth aspect of the invention, the image forming means preferably includes a plurality of image bearing members, and a sheet material sequential conveyance means for sequentially conveying the sheet material to the plurality of transfer nip portions as the sheet material supporting member, wherein toner images respectively borne by the plurality of image bearing members are sequentially transferred to the sheet material conveyed by the sheet material sequential conveyance means.
In an eleventh aspect of the invention, the image formation driving source preferably includes different image formation driving sources for respectively driving the plurality of image bearing members and the sheet material sequential conveyance means.
In a twelfth aspect of the invention, the image forming apparatus preferably further comprises a fixing means which holds and conveys a sheet material to which an unfixed toner image has been transferred to thereby fix the toner image to the sheet material, wherein the fixing means is driven by the image formation driving source, and conveys the sheet material at a speed which is substantially the same as the speed at which the sheet material is conveyed by the image forming means.
In a thirteenth aspect of the invention, there is provided an image forming apparatus comprising: an image forming means for conveying a sheet material and forming an image on a sheet material; a conveying means for conveying the sheet material toward the image forming means; an image formation driving source for driving the image forming means; a conveyance system driving source for driving the conveying means; and a control means which makes the speed at which the sheet material is conveyed by the image forming means variable and controls the conveyance system driving source and the image formation driving source such that the speed at which the sheet material is conveyed by the conveying means is substantially the same as the speed at which the sheet material is conveyed by the image forming means when the image forming means forms an image.
In a fourteenth aspect of the invention, it is desirable that the control means controls the conveyance system driving source such that the sheet material is conveyed at a speed higher than the speed at which the sheet material is conveyed by the image forming means before the sheet material is conveyed by the image forming means.
In a fifteenth aspect of the invention, it is desirable that the control means has the function of varying the speed at which the sheet material is conveyed by the image forming means according to the kind of the sheet material.
In a sixteenth aspect of the invention, it is desirable that the image forming means includes: a plurality of image bearing members for bearing a different color toner image, and a conveying belt for conveying the sheet material, wherein the sheet material is conveyed between the image bearing members and the conveying belt by the conveying belt.
In a seventeenth aspect of the invention, it is desirable that the image forming means further includes a fixing means for fixing an unfixed image to the sheet material, and the image formation driving source for driving the image bearing members, the conveying belt, and the fixing means.